Diamond provides numerous superior characteristics such as high thermal conductivity, high electron/hole mobility, high dielectric breakdown field, low dielectric loss and wide band gap, and nothing else is similar as a semiconductor material. Specifically, electric field transistors with high frequency characteristics have recently been developed, and are expected to be used as power device semiconductors. Diamond has a negative electron affinity, and research on practical use as an electron emitter element is in progress. Regarding optical characteristics, diamond also has high transmissivity in the ultraviolet region (225 to 400 nm) and a high refractive index. For these reasons, it is anticipated that diamonds will be used as pick up lens materials that can accommodate the increasingly shorter wavelengths in conjunction with the increasingly high density of optical disks, etc.
It is necessary to have large-scale single crystal substrate equal to that of other semiconductor materials in order to use diamond as a semiconductor or an optical material. This is because semiconductor wafer processing and the equipment thereof, which are required for manufacturing devices, have been designed with assuming wafers with a diameter of several inches. Currently, high-temperature, high-pressure synthesis and chemical vapor deposition (CVD) are being industrially developed as methods to obtain large-scale diamond single crystals, and a large-scale substrate having a diameter of about 10 mm in a (100) surface can be obtained. However, at the current point in time this does not reach the goal of achieving a large diameter of not less than 1 inch. Specifically, a diamond substrate having a (111) face orientation, which can be doped with phosphorus and easily obtains n-type conductivity, is a several mm angle size in a mass-production commercial product manufactured by high-temperature high-pressure synthesis, and is difficult to make large-scale and maintain satisfactory crystallinity quality during chemical vapor deposition. In contrast, the heteroepitaxial growth that allows diamond single crystal to grow on a comparatively large-scale heterogeneous substrate does not currently have sufficient crystallinity, and practical applications in semiconductors and in optical uses are limited.
In order to resolve these problems, for example, Japanese Patent Publication No. H8-208387 disclosed an example of a diamond component that combined the advantages of both single crystalline diamond with satisfactory crystallinity and polycrystalline diamond for obtaining a large surface area by surrounding single crystalline diamond having a surface of 1 mm2 or more with polycrystalline diamond.
The diamond part of Japanese Patent Publication No. H8-208387 is mainly intended for sensors and optical windows, etc., and it is necessary to grow high quality single crystals on single crystal substrate. Because (100) single crystal is used as seed crystal in order to grow high quality single crystals, it is difficult to form an n-type layer on this substrate by phosphorus doping. Moreover, in the example of arranging single crystal substrate on a flat silicon substrate, the film formation time in order to achieve sufficient bonding required 220 hours, and this is a disadvantage for productivity. Further, in an example in which a silicon substrate was processed to form a concave and a single crystalline diamond substrate is embedded into the recess processed, the heights of the silicon substrate main face and of the single crystalline diamond substrate main face coincide. Because the heights of both surfaces coincide, it is not possible to completely remove the chemical vapor deposition single crystalline diamond layer on the single crystalline diamond substrate by polishing, etc., and it is not possible to expose and utilize a single crystalline diamond substrate part with satisfactory crystallinity, either.
Moreover, single crystalline diamond substrates indicated in Japanese Patent Publication No. H8-208387 are insulative type IIa and type Ib diamonds at room temperature, are undoped single crystalline diamonds, and cannot be used as semiconductor devices as is. Because the silicon substrate exemplifying the diamond part substructure is also insulative at room temperature, it is not possible to use in a sensor, etc. unless the silicon substrate is removed and electrodes and conductive diamond films are formed again. The insulative diamonds, as explained in Japanese Patent Publication No. H8-208387, are processed by a laser cutting, but the processing speed poses difficulties. Further, in order to arrange a single crystalline diamond substrate on a silicon substrate and to cover over this with polycrystalline diamond, it is necessary to form sufficiently thick polycrystalline diamond, and the throughput has difficulties.